The present invention relates to an electrophotograpic image recording apparatus and, more particularly, to an image recording apparatus of the type driving optics in response to multi-level image data to electrostatically form a latent image on an image carrier.
An electrophotographic image recording apparatus such as a copier or a laser printer has optics or an image reader for reading an image to be recorded. Image light representative of the read image scans a photoconductive element, or image carrier, to electrostatically form a latent image thereon. A developing unit develops the latent image to produce a toner image. The toner image is transferred to a paper sheet, or recording medium, and then fixed on the sheet by a fixing unit. Regarding a photograph or similar halftone image, it has been customary for an image processing section incorporated in this type of apparatus or a host computer to produce quasi-halftone image data from two-level data by use of a dither method or a density pattern method. The apparatus drives the optics in response to such quasi-halftone image data to thereby form an electrostatic latent image on the photoconductive element. Specifically, while the writing operation of the optics is based on two levels per pixel, halftone is rendered by adequately distributing pixels on which a toner deposits and pixels on which it does not deposit. The density is determined by the ratio of the pixels with a toner to the entire pixels. This kind of approach is successful in reducing the volume of image data and, therefore, in promoting efficient image transfer and storage. However, an increase in the number of tones directly translates into a decrease in resolution and undesirably highlights particular patterns in a reproduction. It is therefore difficult to enhance reproducibility in both of two different kinds of image areas, i.e., a character or similar area that needs high resolution and a photograph or similar area that needs faithful representation of tones.
Another conventional approach for rendering halftone consists in varying the exposing level of a semiconductor laser included in the optics in multiple steps by power modulation or varying the emission time thereof in multiple steps by pulse width modulation. If this kind of approach can control the tones of an image, the problem discussed above will be eliminated. However, the number of exposing levels available with the power modulation scheme, i.e., the modulation of a current for driving the laser is limited. Specifically, the relation of the optical attenuating characteristic of the photoconductive element and the laser drive current to the emission intensity is apt to change due to the changes in ambient conditions and deterioration of the photoconductive element, etc. Then, when a great number of exposing levels are set, the relation between the tones of input image data and the tones of a latent image to be formed on the photoconductive element will be noticeably changed. On the other hand, when the emission time is changed in multiple steps by modulating the pulse width, the potential of a latent image changes, resulting in the same phenomenon as stated in relation to power modulation. This prevents the image recording characteristic from being unconditionally determined and thereby renders the reproducibility of tones unstable.
There has also been proposed a method which measures the intensity of a laser beam issuing from a semiconductor laser and feeding back the measured intensity to the current which drives the laser. However, with a conventional apparatus implemented with this method, it is difficult to measure a tone represented by a particular amount of exposure. This kind of scheme would fail to suppress the fluctuation of optical attenuation characteristic, also resulting in an unstable recording characteristic. While Japanese Patent Laid-Open Publication Nos. 204741/1989 and 204743/1989 each shows and describes a control method relying on tone correction, they are not of the type detecting the image forming conditions of an image recorder and, therefore, not capable of eliminating the problems stated above.